Your assessment on the drop-in replacement for STK10C68-5C45M is correct. The STK10C68-5C45M is obsolete and unfortunately we don't have any pin to pin drop-in replacement for the same density, package, and MIL qual option. The closest nvSRAM density option available is 256Kb in SOIC and TSOP packages which are different from 28-CDIP offered in STK10C68-5C45M. You can access the 256Kb datasheet at the following link.